A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus may include an optical measurement system arranged on a substrate table that is configured to hold the substrate. An example of such an optical measurement system is a device for image position detection used to align a reticle to the substrate table. The reticle is therefore provided with a structure (e.g. a grating) and the substrate table is provided with a complementary structure. By transmitting a radiation beam through the structure on the reticle and the complementary structure on the substrate table and detecting the image by a sensor part on the substrate table, the position and focus of the image can be determined. This position and focus information can be sent to a position control unit or controller of the substrate table which is able to align the reticle to the substrate table based on the information. Instead of transmitting a radiation beam through the complementary structure, the radiation beam may be reflected of the complementary structure.
In case a projection system is used to project the patterned radiation beam onto a target portion of the substrate, an optical measurement system can be provided that is configured to characterize the projection system. Properties and parameters that can be measured are for instance aberrations, the detailed shape of the pupil distribution and/or the transmission (apodisation). The measurement system may include a grating on the substrate table to split a converging wavefront at the convergence point into multiple laterally sheared copies of this wavefront. Interference of the sheared wavefronts is then observed at a sensor part.
With conventional optical measurement systems, electrical wires/cables are needed to power the systems and to transfer their measured data to data processing units. Further, cooling may be required to cool the electronics on the substrate table. The latter may in particular be applicable for EUV lithographic apparatus. This all leads to disturbance forces and torques acting on the substrate table, reducing positioning accuracy and hence overlay and imaging performance.